1. Field of the Invention
The present invention relates to a doctor blade support used in silk screen printing machines.
In particular, the invention relates to a doctor blade support intended to print a silver- or aluminium-based conductor paste on a support, a substrate or a silicon wafer.
2. Description of the Related Art
Such printing machines are used in the field of micro-electronics to produce printed circuits and in the field of the manufacture of solar wafers for the production of solar panels.
Conductor pastes must be printed regularly at a constant thickness in order not to impair the electrical characteristics of the circuits or solar panels so formed.
In addition, the pattern to be printed must be reproduced with precision.
As a strong pressure is necessary to transfer the conductor paste from the screen to the element to be printed, the doctor blade support must have a high degree of rigidity.
In order to produce such printing, a doctor blade composed of synthetic elastomeric material is secured in a metal support.
The doctor blade has a rectangular parallelepipedal shape of substantially square cross-section. It is generally obtained by cutting a strip from a sheet of elastomeric material. This cutting operation brings about variations in the dimensioning of the square cross-section greater than one tenth of a millimeter.
For printing, the doctor blade support is displaced in accordance with a translational movement above a screen for silk screen printing placed above the element to be printed. A pressing force is exerted on the doctor blade during its displacement in order to force the conductor paste to pass through the open meshes of the screen in order to deposit a pattern to be printed on the element to be printed.
In order to print conductor pastes in a uniform manner, and with a high degree of precision, in particular the U.S. Pat. No. 3,482,819 has proposed a doctor blade support carrying a doctor blade of synthetic elastomeric material having a square cross-section, one of the corners of which forms the surface in contact with the screen. This doctor blade is clamped between two metal clamping jaws reproducing the shape of the square cross-section of the doctor blade. Only a small portion of the corner of the doctor blade projects relative to the two clamping jaws. Generally, the doctor blade is clamped in the clamping jaws in such a manner that one of the diagonals of the square cross-section of the doctor blade is in a vertical plane, so that the angle formed between the surface of the screen and one of the faces of the doctor blade is equal to 45°.
The document U.S. Pat. No. 4,122,771 proposes a variant of that doctor blade support in which the angle formed between the surface of the screen and one of the faces of the doctor blade is equal to 30°.
In the documents U.S. Pat. No. 3,482,819 and U.S. Pat. No. 4,122,771, the clamping jaws of the doctor blade supports are secured to each other by several screws aligned in the longitudinal direction of the clamping jaws.
The clamping jaws of the doctor blade supports may also be secured to each other by a single tightening handle as described, for example, in the document U.S. Pat. No. 5,458,060.
The doctor blade obtained does not always have a completely uniform cross-section along the entire parallelepiped, so that, after the tightening of the tightening handle described in the document U.S. Pat. No. 4,122,771, or even of the screws of the doctor blade supports described in the documents U.S. Pat. No. 3,482,819 and U.S. Pat. No. 4,122,771, the fine edge of the corner of the doctor blade is not completely flat and straight. It has protuberances at the sites where the area of the cross-section of the doctor blade is largest. For, at those sites, the force exerted by tightening has the effect of causing the fine edge of the corner of the doctor blade to bulge by compression of the elastomeric material. Of course, in order to avoid such a phenomenon, all that is required is to tighten to a slightly lesser extent at those sites.
In practice, the tightening is adjusted in accordance with the straightness of the fine edge of the corner of the doctor blade by placing the doctor blade on a surface plate before it is mounted in the machine. Such a regulation of the tightening may involve major time losses, the duration of this regulation being in accordance with the tolerances in the cross-section of the doctor blade.